A team of nanotechnologists at The University of Texas at Dallas, along with Brazilian collaborators, have discovered that sheets of carbon nanotubes can produce bizarre mechanical properties when stretched or uniformly compressed.

These unexpected but highly useful properties could be used for such applications as making composites, artificial muscles, gaskets or sensors. The team’s findings are reported in the April 25 issue of the journal Science.

When most materials are pulled in one direction, they get thinner in the other direction, similar to how a rubber band behaves when it is stretched. However, specially designed carbon nanotube sheets, dubbed “buckypaper,” can increase in width when stretched. The buckypaper can also increase in both length and width when uniformly compressed.

Ordinary materials contract laterally when stretched — a phenomenon that can be quantified by Poisson’s ratio, which is the ratio of the percent lateral contraction to the percent applied stretch.

Dr. Ray H. Baughman, Robert A. Welch Professor of Chemistry and director of UT Dallas’ NanoTech Institute, and his colleagues created their nanotube sheets, or buckypaper, by drying a fibre slurry. The slurry has a mixture of carbon single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). The researchers found that increasing the amount of MWNTs in the paper produced a sharp transition from a positive Poisson’s ratio of about 0.06 to a much larger magnitude negative value of about -0.20.

As described by the team in Science, this transition can be understood by relating the deformation modes of the nanotube sheets to those of a collapsible wine rack. If two neighbouring nanotube layers are coupled like the struts in a compressible wine rack, Poisson’s ratio is positive and the rack becomes narrower when stretched. In contrast, if the rack is blocked so that it can no longer be collapsed but the struts are stretchable, increases in strut length produce a negative Poisson’s ratio.

“This abrupt switching of the sign of Poisson’s ratio is so surprising and the structure of the nanotube sheets is so complicated that we initially believed that quantitative explanation was impossible using state-of-art theoretical capabilities,” said Baughman, the article’s corresponding author. “Distant daily teaming with our Brazilian colleagues through the Internet enabled us to jointly extract essential features from a structure that was much too complex for complete analysis, leading to our successful wine-rack-like model.”

Baughman and his team subsequently found that the nanotube sheets containing both single-walled and multi-walled nanotubes had a 1.6 times higher strength-to-weight ratio, 1.4 times higher modulus-to-weight ratio and a 2.4 times higher toughness than sheets made of SWNTs or MWNTs alone.

According to Baughman, the implications of the discovery that properties can be enhanced by mixing nanotube types can likely be extended from nanotube sheets to other nanotube arrays, like the twisted nanotube yarns Baughman and colleagues invented in 2005.